The development of materials for gearboxes is subject to many demands, some of them mutually contradictory. On the one hand, the gears must be effectively lubricated, i.e. the coefficient of friction between them must be as low as possible, while on the other hand, the synchronizing rings must have a high coefficient of friction which remains constant independently, in particular, of the temperature, the speed and the pressure.
One suggestion is to cover the active surface of the synchronizing rings with a suitable material such as molybdenum. This method is expensive.
Another technique is aimed at preventing an oil layer from forming, or causing the oil film to break by creating geometric irregularities by machining grooves or the like or by means of finer heterogeneities by using a non-homogeneous material, in particular a relatively soft matrix containing harder particles. Nevertheless, these friction materials have until now given results which vary according to the conditions under which they are used.
The studies carried out have lead to the conclusion that these materials could provide good results, at a relatively low cost, if certain conditions are fulfilled.